Evolution of models describing hematopoiesis. (A) Classic hierarchical model of hematopoiesis. (B) Revised model of hematopoiesis with LMPPs and BMCPs. (C) Differentiation landscape model of hematopoiesis. Hematopoietic stem and progenitor cells differentiate along ≥1 trajectories in the landscape. Each arm represents the entry point to 1 lineage, with the exception of the HSC arm, which is the starting point. Previously described bi- and multipotent progenitors are indicated, but the model is flexible and allows for tripotent erythroid, mast cell, and basophil progenitors, for example. This model focuses on myeloerythroid cell differentiation; the multiple lymphoid populations that exist are therefore not described in detail. Ba, basophil; CLP, common lymphoid progenitor; Eo, eosinophil; Ery, erythrocyte; HSC, hematopoietic stem cell; lymph, lymphocyte; MC, mast cell; Meg, megakaryocyte; mono, monocytes; MPP, multipotent progenitor; Neu, neutrophil.